In a polishing apparatus for polishing a substrate, such as a semiconductor wafer, an in-situ spectral film-thickness monitor is used for the purpose of mainly monitoring a progress of polishing of a dielectric layer (transparent layer) and detecting a polishing end point. This in-situ spectral film-thickness monitor has a light source and a spectrophotometer mounted to a polishing table. The light source and the spectrophotometer are connected to a light-transmitting fiber and a light-receiving fiber, respectively. Distal ends of these fibers, which serve as a light-transmitting element and a light-receiving element, are arranged at positions as to scan a wafer surface once each time the polishing table rotates. The light-transmitting element and a light-receiving element are located so as to sweep across the center of the wafer, so that, each time the polishing table rotates, the light-transmitting element and the light-receiving element scan the wafer surface in a line (curved line) that is approximate to a diameter of the wafer surface.
In recent years, as semiconductor devices have been becoming smaller and finer, there is an increasing need for an improved polishing performance for a more-precise polishing result, and therefore there is a very stringent need for a precision of the in-situ spectral film-thickness monitor. However, since the in-situ spectral film-thickness monitor is not configured to obtain an absolute value of a film thickness, a measured value of the film thickness deviates slightly from a measured value of the film thickness obtained by an in-line (or a stand-alone) film-thickness measuring device that has been calibrated based on a film thickness of a reference wafer.
Moreover, it is not easy to calibrate the in-situ, spectral film-thickness monitor, because the in-situ spectral film-thickness monitor is installed in the polishing table. In other words, it is a time-consuming operation to calibrate the in-situ spectral film-thickness monitor that is installed in the polishing table, and there is a limit in a space for installing an automatic calibrating device. Further, components of the in-situ spectral film-thickness monitor themselves may be deteriorated with time. In addition, measurement points on the wafer surface and a distribution thereof are different from those of the in-line film-thickness measuring device. Under such circumstances, the measured values of the film thickness obtained by the in-situ spectral film-thickness monitor do not always agree with the measured values of the film thickness obtained by the in-line film-thickness measuring device. In addition, if a thickness of a layer that lies underneath a film to be polished varies from wafer to wafer, detection results of the polishing end point may also vary from wafer to wafer due to the influence of the variation in the thickness of the underlying layer.
Further, there may be a variation in the film thickness along a circumferential direction of the wafer. Such a variation in the film thickness along the circumferential direction of the wafer may adversely affect the measurement of the film thickness over the entire wafer.
In order to reduce the influence of the variation in the film thickness, there is a proposed method in which rotational speeds of a polishing table and a top ring are adjusted appropriately (see Japanese laid-open patent publication No. 2010-240837). According to this method, a film-thickness sensor scan a wafer surface in its entirety and can therefore obtain an average of the film thickness. However, in order to obtain the average of the film thickness, it is necessary to obtain film thickness data until the polishing table rotates multiple times. As a result, a time delay in monitoring of the film thickness may occur, and excessive polishing or insufficient polishing may occur due to a possible change in polishing rate.